Icirrus City is where diamond dust is supposed to happen in the Unova region. Unfortunately, there isn't a list of confirmed dates for the 5th gen games. Because of the seasons system in Black, White, Black 2, and White 2, it's safe to assume that diamond dust will only happen in the winter months (December, April, and August). We are in a winter month in the 5th gen right now, and if the diamond dust is going to happen in BW2, December 25 and December 31 are really good bets. Keep an eye out for it at Icirrus City, because those two dates are fast approaching. (In BW2, you get a medal for seeing diamond dust, so if you want to complete your medal collection, here's your chance to get this time-restricted medal).